1. Field of the Invention
This invention relates to a leak detector of the type commonly used for remote submersible petroleum pumping systems, and includes a valve interposed in a fluid conduit which is shiftable by means of a pressure sensing piston received in a chamber communicating with the fluid conduit, for retaining the valve in its reduced flow position when leakage from the conduit exceeds a certain, predetermined value. The face of the piston has walls and the piston chamber has structure which cooperates with the walls to present a reduced area region when the valve is in the reduced flow position, and the piston wall disengages from the chamber structure when the valve is in the full flow position to enable the detector to compensate for greater quantities of thermal contraction of fluid in the conduit without causing the valve to return to its first, reduced flow position.
2. Description of the Prior Art
For years, detectors have been provided for sensing leakage from buried piping of a service station due to the inherent hazards of the petroleum products and the fact that such leakage could otherwise go unnoticed for exended periods of time. Typically, such detectors are positioned adjacent the buried storage tanks of the service station and monitor the downstream pressure of gasoline in the lines between the tank and a service station island or dispenser valve.
One type of leak detecting apparatus is disclosed in U.S. Pat. No. 3,454,195, issued July 8, 1969 and owned by the assignee of the present invention. In order to facilitate an understanding of the present invention, the disclosure of U.S. Pat. No. 3,454,195 is expressly incorporated into the present disclosure. Additionally, a piston leak detector identified as the Red Jacket Two-Second Leak Detector is described in some detail in Brochure No. 5170 6/80 from Red Jacket Pumps of Davenport, Iowa, and the disclosure of this brochure is also hereby expressly incorporated into the present disclosure.
In brief, the leak detectors described in U.S. Pat. No. 3,454,195 and Red Jacket Brochure No. 5170 6/80 include a diaphragm operated valve to sense gasoline leakage. When a pump adjacent the storage tank is energized, the diaphragm senses pressure in the piping and, if no leaks are present, fluid pressure in the piping will shift the diaphragm and a valve poppet coupled to the diaphragm to enable a full flow of gasoline. On the other hand, if a leak exists, fluid pressure cannot build to a value sufficient to effect complete shifting of the diaphragm and poppet, and full flow of gasoline is thus precluded.
Unfortunately, excessive thermal contraction of gasoline in the piping can lower fluid pressure against the diaphragm to such a value that the diaphragm shifts the valve poppet to prevent full flow when the dispenser valve is held open even though no leakage is present. Thermal contraction of gasoline in the piping is particularly apparent during the fall and spring seasons when ground temperatures are subject to relatively wide fluctuations in temperature. Gasoline that is pumped from a warm, relatively large underground storage tank to cold piping located a few feet below the surface of the ground can contract significantly and close the valve, causing frustration to the consumer attempting to dispense gasoline into the tank of a vehicle if the nozzle is held open during the leak detecting phase.
In order to overcome the problem of thermal contraction of gasoline in service station piping, time delay relays and solenoids are sometimes provided to prevent dispensing of gasoline at the island until a sufficient amount of time has elapsed from the time of pump actuation. However, such a procedure represents extra initial costs and added labor and expense for maintenance.
Other leak detectors are known in the art which utilize a piston instead of a diaphragm for sensing pressure of fluid within the piping and for shifting a valve between a reduced flow, metering position and a full flow position. In general, piston leak detectors are somewhat more satisfactory than diaphragm leak detectors for compensation of thermal contraction since the volume of fluid in the chamber adjacent the pressure-receiving face of the piston can be readily increased without significant mechanical problems to a relatively large volume so that contraction of the gasoline does not shift the valve poppet to any significant extent. However, when relatively large piston chambers are employed, movement of the piston when the poppet is in the leak sensing position adjacent a metering pin, is correspondingly reduced and thus a larger amount of time is ncessary for shifting the piston and poppet away from the metering pin to enable full flow. As can be appreciated, leak detectors which complete the leak sensing cycle in a time period of greater than two or three seconds are commercially undesirable.